Spur tube heat exchanger



Nov. 8, 1966 J. F. HARVEY 3,283,811

SPUR TUBE HEAT EXCHANGER Filed Sept. 8, 1964 INVENTOR. John F. Harvey AT TOP NEY United States Patent 3,283,811 SPUR TUBE HEAT EXCHANGER John F. Harvey, Akron Ohio, assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Sept. 8, 1964, Ser. No. 394,803 7 Claims. (Cl. 165-442) This invention relates generally to heat transfer apparatus having heat transfer elements for boilers and super-heaters. More particularly, the invention relates to improved heat transfer element-s known in the art as bayonet tubes or field tubes. These may comprise an upright outer tube which is closed at its lower end and contains a second concentric tube, down which a heat-absorbing fluid passes, and then returns up the an nular space between the two tubes.

The heat transfer elements are usually dipsosed in a hot gas passageway, the hot gas giving up heat to heat-abso-rbing fluid circulating through the tubes. Preferably, the annular space between tubes is uniform and the heatabsorbing fluid media flows at high velocity at the point of flow reversal in the element where the highest temperature is experienced, all to the end of carrying away the heat applied to the blind end of the element and thereby maintaining safe temperatures in the metal tubes.

According to the present invention, improved effectiveness in a heat transfer element is achieved by providing projections which accurately position the inner end of the inner tube in spaced and concentric relation to the closed end of the outer tube, and also by providing a restricted flow passageway between the tubes at the blind end of the element in order to increase flow velocity of the heat-absorbing fluid at that location.

The various objects, features, and advantages of the invention will appear more fully from the detailed description which follows, taken in connection with the accompanying drawings forming a part of the present application, and in which:

FIG. 1 is a sectional view of a gas heated boiler including tubular heat transfer elements constructed in accordance with the invention, showing parts in elevation;

FIG. 2 is an enlarged fragmentary view, partly in section and partly in elevation, of the lower end portion of a tubular heat transfer element; and

FIG. 3 is a view, partly in section and partly in elevation, looking upwardly into the lower end of the tubular heat transfer element of FIG. 2.

As shown in the drawings, the numeral generally designates an upright heat transfer apparatus including a hot gas passageway or flue 12 disposed between an outer shell 14 and an inner shell 16. A horizontal tube sheet 1 8 is secured, as by bolts 20, to a flange 22 of the outer shell 14 to close one end of the hot gas flue 12. Another horizontal tube sheet 24 is secured by bolts 26 to an inwardly directed flange 28 of wall structure 29. Disposed between the tube sheet 24 and the tube sheet 18 is an outlet chamber 30 for the heat-absorbing fluid; and on the upper side of the tube sheet 24, adjacent the wall structure 29, is an inlet region 32 for the heat-absorbing fluid.

Each tubular heat transfer element of the present in- VentiOn is generally designated by the numeral 34. As shown, each element 34 comprises vertically elongated inner and outer tubes designated by the numerals 36 and 38, respectively. These tubes are disposed one within the other, in concentric, spaced relationship. The outer tube 38 is, of course, substantially larger in cross section than the inner tube 36; it is closed at the lower end thereof; .and the upper end thereof opens to the outlet chamber 30. The upper end of the outer tube 38 is received in vertically elongated holes in the tube I er.

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sheet 18 and it can be secured in any well-known manner, such as by rolling or welding; however, as shown in the drawings, the connection is preferably made by welding. In order to minimize side-to-side movement of the outer tubes 38 relative to one another, a brace member 40 may be employed to connect the tubes rigidly together, as shown.

The inner tube 36 is concentrically positioned within the outer tube 38, and with respect to the longitudinal axis of the assembly. Both the upper and lower ends of the inner tube 36 are open, with the lower end thereof terminating adjacent the closed end of the outer tube 38. The upper end portion of the inner tube 36 passes through the outlet chamber 30 without communicating therewith then through the outer tube sheet 24, and finally terminating with its open end in the inlet region 32. A swage lock fitting 41 at the upper end of the inner tube 36 is employed to effect a tight seal between the tube 36 and the tube sheet 24; and also to ensure that the inner tube 36 is secured against longitudinal movement relative to the outer tube 38.

With the arrangement as set forth, the inner tube 36 defines therewithin a first passageway 42 in which .a heatabsorbing fluid is conducted downwardly along the length thereof from the inlet region 32 toward the lower end portion of the heat transfer element 34, at which point a reversal in the direction of flow is effected and the heatabsorbing fluid flows into the outlet chamber 30 via a second passageway 43 of annular cross section disposed between the tubes 36 and 38. Stated another way, the second passageway 43 is annul'arly dis-posed about the first passageway 42, in counterflow relationship therewith.

The heat-absorbing fluid serves to carry away heat, acting as a coolant, at the same time is is superheated by hot gas flowing through the flue 12 at 1700" F. and high Therefore, the tube materials of element 34 should be capable of enduring such temperatures and, in the interest of preventing damage to the tubes 36 and 38, a large amount of heat must be carried away, by the heatabsorbing fluid flowing therethrough. Also, the tubes 36 and 38 must be maintained in concentric relationship to avoid hot spots in the tubes and to provide symmetrical flow areas. An increase in fluid flow velocity within the lower end portion of the element 34 has been found to improve the efliciency of heat transfer betwee the fluid and the lower end portion of the outer tube 38.

The foregoing can be accomplished with greater effectiveness by providing a tubular fl-ow fitting 44, to be de scribed, at the lower end of the inner tube 36. It is the function of the flow fitting 44 to ensure concentrici-ty of the tubes, and to provide an upwardly curving, restricted passageway 45, disposed between the lower ends of the tubes 36 and 38, which connects the passageways 42 and 43. The flow fitting 44 is positioned in coaxial relationship with the lower end of the inner tube 36 and secured thereto by cooperating threads 46.

As shown in FIG. 2, the downwardly tapering lower end portion 47 of the outer tube 38 is of hollow hemispherical construction. In spaced and mutually facing relationship with this lower end portion 47 of tube 38 is a rounded tip of the downwardly tapering, lower end portion 48 of flow fitting 44. The restricted passageway 45 defined by these mutually facing surfaces reverses the direction of fluid flow at the blind lower end of the element 34.

The lower end of the fitting 44 and the closed lower end of the tube 33 are maintained in spaced, concentric relationship by several projections 50, preferably tour, which extend downwardly and radially outwardly from the lower end of the fitting 44 into engagement with the closed lower end of the tube 38. The projections 54 3 are coextensive along the common axis of the tube 36 and the fitting 44.

Still referring to FIG. 2, the flow fitting 44 has an internal surface, the upper portion of which is threaded at 46 for cooperation with external threads on the lower end of the inner tube 36, and the lower portion of which tapers downwardly to provide a downwardly converging extensio 51 of the passageway 42. This downwardly convergent extension 51 of passageway 42 provides a gradual reduction in fiow area for the purpose of increasing the velocity of the fluid flowing therethrough. It will be appreciated that gradual changes in flow area are preferred to abrupt changes in order to minimize energy losses due to friction.

The exterior of the fiow fitting 44 is generally keg shaped, comprising a downwardly tapering lower surface portion 54, an upwardly tapering upper surface portion 56, and a substantially cylindrical intermediate surface portion 58.

The upper surface portion 56 defines with the internal surface of the outer tube 38 an upwardly diverging inlet extension 59 to the passageway 43 for reducing the velocity of fluid which is traveling upwardly after reversing direction in passageway 45.

The apparatus of the present invention is assembled by first inserting the upper end of each outer tube 38 into the vertical elongated openings in the tube sheet 18, and then welding the upper ends thereto. Next, a flow fitting 44 is threaded to the lower end of its associated tube 36, after which the inner tube assembly is inserted through the tube sheet 24, and then into the :outer tube 38 until the projections 50 engage the lower end portion of the outer tube 38. The securing of the swage lock fitting 41 to the tube sheet 24 exerts a downward bias which is sufi icient for the fitting 44 to center itself within the lower end portion 47 of the outer tube 38, thereby establishing the desired concentric relationship, and at the same time the projections 50 maintain the end spacing which defines the flow reversal passageway 45. Furthermore, the associated openings in the tube sheets 18 and 24- are vertically aligned, thus ensuring coaxial alignment of the tubes 36 and 38 received therein.

In operation, heat-absorbing fluid media, such as water, enters the upper end of the inner tube 36 from the inlet region 32, flows to the lower end of tube 36, and increases in velocity as it passes through the convergent extension 51 of passageway 42. Next, the heat-absorbing fluid impinges forcefully on the internal surface at the lower end portion 47 of the outer tube 33, with the result that this region of passageway 45 is flushed free of foreign matter; and, after reversal of fiow direction and absorption of heat in passageway 45, the fiuid expands through the upwardly divergent inlet passageway 59 to the passageway 43, and thereafter continues fiowing upwardly while continuing to absorb heat from the wall of the tube 38, finally discharging into the outlet chamber 30.

From the foregoing it can be seen that an improved heat transfer element has been provided for use in heat transfer apparatus. The above described flow fitting 44 maintains the inner tube structure of a field tube in proper longitudinally and radially spaced, concentric relationship for ensuring symmetrical annular flow through balanced flow areas of the heat transfer element 34. This is accomplished simply and effectively, at small cost, and with improved results, namely, accurate sizing of the flowreversal passageway 45, protection of the tube metals from overheating, and prevention of accumulation of foreign matter.

Although the invention has been shown in but one form, it will be obvious to those skilled in the art that it is not so limited, but that it is susceptible of various changes and modifications without departing from the spirit there- .of as set forth in the following claims.

What is claimed is:

1. A heat transfer element including inner and outer concentric elongated tubes which are open and closed, respectively, at one end of said element, a tubular fitting on said one end of the inner tube and defining therewith a first passageway which converges within said fitting in a first direction toward the closed end of said outer tube, there being a second passageway of annular cross section between said inner and outer tubes, said fitting including a plurality of projections, each extending into engagement with the closed end portion of said outer tube, and defining therewith a restricted third passageway which connects said first and second passageways and has substantially less flow area than said first and second passageways.

2. A heat transfer element according to claim 1 wherein said fitting has an exterior surface tapering in a direction opposite to said first direction so as to define with the closed end portion of said outer tube an end section of said second passageway which is divergent in a direction opposite to said first direction.

3. A heat transfer element comprising:

inner and outer tubes having associated open and closed ends respectively disposed one within the other in spaced relationship to define an inner first passag way within an outer second passageway of annular cross section,

the open end portion of said inner tube tapering interiorly in a first direction toward the closed end portion of said outer tube so as to define an end section of said first passageway which is convergent in said first direction and tapering exteriorly in a direction opposite to said first direction so as to define with the closed end portion of said outer tube an end section of said second passageway which is divergent in a direction opposite to said first direction,

and a plurality of annularly spaced projections extending between said end portions for maintaining said end portions in spaced apart and concentric relationship and defining therebetween a restricted third passageway between the end sections of said first and second passageways, said third passageway having a flow area which is substantially less than the fiow areas of said first and second passageways.

4. A heat transfer element according to claim 1 wherein said spaced projections extend from said open end portion of said inner tube toward the closed end portion of said outer tube for maintaining said end portion longitudinally spaced apart and in concentric relationship,

and including means securing said tubes against longitudinal movement relative to one another.

5. A heat transfer element comprising:

inner and outer elongated tubes having respective associated open and closed ends spaced and arranged to define a first passageway concentrically positioned within a second passageway of annular cross section, the open end portion of said inner tube being tapered interiorly in a first longitudinal direction toward the closed end portion of said outer tube so as to define an end section of said first passageway which is convergent in said first direction and being tapered exteriorly in a direction opposite to said first direction so as to define with the closed end portion of said outer tube an end section of said second passageway which is divergent in a direction opposite to said first direction,

a plurality of spaced projections extending from said open'end portion of said inner tube toward the closed end portion of said outer tube for maintaining said end portions in spaced apart and concentric relationship and defining therebetween a restricted tion of said outer tube for maintaining said fitting and the closed end portion of said outer tube in i predetermined spaced relationship, thereby defining a restricted third passageway between the extensions of said first and second passageways, said third passageway having a flow area which is substantially less than the flow areas of said first and second passageways.

7. A heat transfer element according to claim 6 where- 10 in the closed end of said outer tube tapers in said first a tubular fitting secured to the open end portion of 15 to define a first passageway in said inner tube and a second passageway between said inner and outer tubes, said second passageway being of annular cross section and disposed about said first passageway,

direction, said projections are coextensive in first direction, and including means for biasing said inner tube and the projections carried thereon in first direction.

References Cited by the Examiner said inner tube, said fitting having an internal surface portion tapering in a first direction toward the UNITED STATES PATENTS closed end of said outer tube so as to define an gggi extension of said first passageway which is convergent in said first direction, said fitting having an 20 FOREIGN PATENTS external surface portion tapering in a direction op- 555,986 9 /1943 Great Britain.

posite to said first direction SO as to define with said 17 272 2 1949 Great Britain outer tube an extension of said second passageway which is divergent in a direction opposite to said first direction,

said fitting being provided with a plurality of spaced projections which extend toward the closed end por- ROBERT A. OLEARY, Primary Examiner. A. W. DAVIS, Assistant Examiner. 

1. A HEAT TRANSFER ELEMENT INCLUDING INNER AND OUTER CONCENTRIC ELONGATED TUBES WHICH ARE OPEN AND CLOSED, RESPECTIVELY, AT ONE END OF SAID ELEMENT, A TUBULAR FITTING ON SAID ONE END OF THE INNER TUBE AND DEFINING THEREWITH A FIRST PASSAGEWAY WHICH CONVERGES WITHIN SAID FITTING IN A FIRST DIRECTION TOWARD THE CLOSED END OF SAID OUTER TUBE, THERE BEING A SECOND PASSAGEWAY OF ANNULAR CROSS SECTION BETWEEN SAID INNER AND OUTER TUBES, SAID FITTING INCLUDING A PLURALITY OF PROJECTIONS, EACH EXTENDING INTO ENGAGEMENT WITH THE CLOSED END PORTION OF SAID OUTER 